Hazardous material repository systems and methods

ABSTRACT

A power generator system includes one or more heat transfer members configured to contact: a heat source in a hazardous waste repository of a directional drillhole that stores nuclear waste in one or more nuclear waste canisters, and a heat sink in the hazardous waste repository; and one or more thermoelectric generators thermally coupled to the one or more heat transfer members and configured to generate electric power based on a temperature difference between the heat source and the heat sink.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of, and claims priority under 35U.S.C. § 120 to, U.S. patent application Ser. No. 17/001,057, filed onAug. 24, 2020, which claims priority under 35 U.S.C. § 120 to, U.S.patent application Ser. No. 16/796,821, filed on Feb. 20, 2020, now U.S.Pat. No. 10,751,769, which in turn claims priority under 35 U.S.C. § 119to: U.S. Provisional Patent Application Ser. No. 62/808,565, filed onFeb. 21, 2019; U.S. Provisional Patent Application Ser. No. 62/808,623,filed on Feb. 21, 2019; U.S. Provisional Patent Application Ser. No.62/808,791, filed on Feb. 21, 2019; U.S. Provisional Patent ApplicationSer. No. 62/808,813, filed on Feb. 21, 2019; and U.S. Provisional PatentApplication Ser. No. 62/833,106, filed on Apr. 12, 2019. The entirecontents of each of the previous applications are incorporated byreference herein.

TECHNICAL FIELD

This disclosure relates to hazardous material repository systems andmethods.

BACKGROUND

Hazardous material, such as radioactive waste, is often placed inlong-term, permanent, or semi-permanent storage so as to prevent healthissues among a population living near the stored waste. Such hazardouswaste storage is often challenging, for example, in terms of storagelocation identification and surety of containment. For instance, thesafe storage of nuclear waste (e.g., spent nuclear fuel, whether fromcommercial power reactors, test reactors, or even high-grade militarywaste) is considered to be one of the outstanding challenges of energytechnology. Safe storage of the long-lived radioactive waste is a majorimpediment to the adoption of nuclear power in the United States andaround the world. Conventional waste storage methods have emphasized theuse of tunnels, and is exemplified by the design of the Yucca Mountainstorage facility. Other techniques include boreholes, including verticalboreholes, drilled into crystalline basement rock. Other conventionaltechniques include forming a tunnel with boreholes emanating from thewalls of the tunnel in shallow formations to allow human access.

SUMMARY

In an example implementation, a power generator system includes one ormore heat transfer members configured to contact: a heat source in ahazardous waste repository of a directional drillhole that storesnuclear waste in one or more nuclear waste canisters, and a heat sink inthe hazardous waste repository; and one or more thermoelectricgenerators thermally coupled to the one or more heat transfer membersand configured to generate electric power based on a temperaturedifference between the heat source and the heat sink.

In an aspect combinable with the example implementation, the nuclearwaste comprises spent nuclear fuel.

In another aspect combinable with any of the previous aspects, the heatsource comprises at least one of the nuclear waste canisters or a casingdisposed in the drillhole.

In another aspect combinable with any of the previous aspects, the heatsink comprises at least one of the casing disposed in the drillhole or amaterial that at least partially fills the drillhole.

In another aspect combinable with any of the previous aspects, thematerial comprises a liquid.

Another aspect combinable with any of the previous aspects furtherincludes one or more biasing members configured to urge the one or moreheat transfer members into thermal contact with the heat source and theheat sink.

Another aspect combinable with any of the previous aspects furtherincludes at least one radiation shield.

In another aspect combinable with any of the previous aspects, theradiation shield comprises tungsten.

In another aspect combinable with any of the previous aspects, the oneor more heat transfer members comprise a radiation resistant material.

In another aspect combinable with any of the previous aspects, the oneor more thermoelectric generators comprise a radiation resistantmaterial.

Another example implementation includes a method for generating power ina hazardous waste repository of a directional drillhole that storesnuclear waste that includes contacting, with one or more heat transfermembers of a power generator system, a heat source in the hazardouswaste repository of the directional drillhole that stores nuclear wastein one or more nuclear waste canisters; contacting, with the one or moreheat transfer members of the power generator system, a heat sink in thehazardous waste repository; and generating electric power with one ormore thermoelectric generators thermally coupled to the one or more heattransfer members based on a temperature difference between the heatsource and the heat sink.

In an aspect combinable with the example implementation, the nuclearwaste comprises spent nuclear fuel.

In another aspect combinable with any of the previous aspects, the heatsource comprises at least one of the nuclear waste canisters or a casingdisposed in the drillhole.

In another aspect combinable with any of the previous aspects, the heatsink comprises at least one of the casing disposed in the drillhole or amaterial that at least partially fills the drillhole.

In another aspect combinable with any of the previous aspects, thematerial comprises a liquid.

Another aspect combinable with any of the previous aspects furtherincludes urging, with one or more biasing members of the power generatorsystem, the one or more heat transfer members into thermal contact withthe heat source and the heat sink.

Another aspect combinable with any of the previous aspects furtherincludes shielding gamma rays generated by the nuclear waste from thepower generator system with at least one radiation shield.

In another aspect combinable with any of the previous aspects, theradiation shield comprises tungsten.

In another aspect combinable with any of the previous aspects, the oneor more heat transfer members comprise a radiation resistant material.

In another aspect combinable with any of the previous aspects, the oneor more thermoelectric generators comprise a radiation resistantmaterial.

Implementations of hazardous waste repository systems and methodsaccording to the present disclosure may also include one or more of thefollowing features. For example, a hazardous waste repository may beused to store hazardous waste material, such as spent nuclear fuel,isolated from human-consumable water sources. The hazardous wasterepository may be suitable for storing the hazardous waste, such asradioactive or nuclear waste, for durations of time up to, for example,1,000,000 years. Other features are described herein.

The details of one or more implementations of the subject matterdescribed in this disclosure are set forth in the accompanying drawingsand the description below. Other features, aspects, and advantages ofthe subject matter will become apparent from the description, thedrawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an example implementation of ahazardous waste repository that includes one or more hazardous materialcanisters according to the present disclosure.

FIG. 2 is a schematic top view of an example implementation of ahazardous waste repository system formed in one or more subterraneanformations that include one or more faults.

FIG. 3 is a schematic illustration of an example implementation of adrillhole seal for a hazardous waste repository according to the presentdisclosure.

FIGS. 4A-4B are schematic illustrations of an example implementation ofa nuclear waste dry cask that encloses one or more nuclear wastecanisters for a hazardous waste repository according to the presentdisclosure.

FIGS. 5A-5B are schematic illustrations of an example implementation ofa power generator system for a hazardous waste repository in adirectional drillhole according to the present disclosure.

FIG. 5C is a graph that illustrates a temperature profile of a hazardouswaste repository in a directional drillhole according to the presentdisclosure.

FIG. 5D is a flowchart that illustrates an example process that includesgenerating electric power with the power generator system of FIG. 5A.

DETAILED DESCRIPTION

FIG. 1 is a schematic illustration of an example implementation of ahazardous waste repository 100 (also referred to as a hazardous wasterepository system), e.g., a subterranean location for the long-term(e.g., tens, hundreds, or thousands of years or more), but retrievable,safe and secure storage of hazardous material (e.g., radioactivematerial, such as nuclear waste which can be spent nuclear fuel (SNF) orhigh level waste, as two examples). For example, this figure illustratesthe example hazardous waste repository 100 once one or more canisters126 of hazardous material have been deployed in a subterranean formation118. As illustrated, the hazardous waste repository 100 includes adrillhole 104 formed (e.g., drilled or otherwise) from a terraneansurface 102 and through multiple subterranean layers 112, 114, 116, and118. Although the terranean surface 102 is illustrated as a landsurface, terranean surface 102 may be a sub-sea or other underwatersurface, such as a lake or an ocean floor or other surface under a bodyof water. Thus, the present disclosure contemplates that the drillhole104 may be formed under a body of water from a drilling location on orproximate the body of water.

The illustrated drillhole 104 is a directional drillhole in this exampleof hazardous waste repository 100. For instance, the drillhole 104includes a substantially vertical portion 106 coupled to a radiussed orcurved portion 108, which in turn is coupled to a substantiallyhorizontal portion 110. As used in the present disclosure,“substantially” in the context of a drillhole orientation, refers todrillholes that may not be exactly vertical (e.g., exactly perpendicularto the terranean surface 102) or exactly horizontal (e.g., exactlyparallel to the terranean surface 102), or exactly inclined at aparticular incline angle relative to the terranean surface 102. In otherwords, vertical drillholes often undulate offset from a true verticaldirection, that they might be drilled at an angle that deviates fromtrue vertical, and inclined drillholes often undulate offset from a trueincline angle. Further, in some aspects, an inclined drillhole may nothave or exhibit an exactly uniform incline (e.g., in degrees) over alength of the drillhole. Instead, the incline of the drillhole may varyover its length (e.g., by 1-5 degrees). As illustrated in this example,the three portions of the drillhole 104—the vertical portion 106, theradiussed portion 108, and the horizontal portion 110—form a continuousdrillhole 104 that extends into the Earth. As used in the presentdisclosure, the drillhole 104 (and drillhole portions described) mayalso be called wellbores. Thus, as used in the present disclosure,drillhole and wellbore are largely synonymous and refer to bores formedthrough one or more subterranean formations that are not suitable forhuman-occupancy (i.e., are too small in diameter for a human to fitthere within).

The illustrated drillhole 104, in this example, has a surface casing 120positioned and set around the drillhole 104 from the terranean surface102 into a particular depth in the Earth. For example, the surfacecasing 120 may be a relatively large-diameter tubular member (or stringof members) set (e.g., cemented) around the drillhole 104 in a shallowformation. As used herein, “tubular” may refer to a member that has acircular cross-section, elliptical cross-section, or other shapedcross-section. For example, in this implementation of the hazardouswaste repository 100, the surface casing 120 extends from the terraneansurface through a surface layer 112. The surface layer 112, in thisexample, is a geologic layer comprised of one or more layered rockformations. In some aspects, the surface layer 112 in this example mayor may not include freshwater aquifers, salt water or brine sources, orother sources of mobile water (e.g., water that moves through a geologicformation). In some aspects, the surface casing 120 may isolate thedrillhole 104 from such mobile water, and may also provide a hanginglocation for other casing strings to be installed in the drillhole 104.Further, although not shown, a conductor casing may be set above thesurface casing 120 (e.g., between the surface casing 120 and the surface102 and within the surface layer 112) to prevent drilling fluids fromescaping into the surface layer 112.

As illustrated, a production casing 122 is positioned and set within thedrillhole 104 downhole of the surface casing 120. Although termed a“production” casing, in this example, the casing 122 may or may not havebeen subject to hydrocarbon production operations. Thus, the casing 122refers to and includes any form of tubular member that is set (e.g.,cemented) in the drillhole 104 downhole of the surface casing 120. Insome examples of the hazardous waste repository 100, the productioncasing 122 may begin at an end of the radiussed portion 108 and extendthroughout the horizontal portion 110. The casing 122 could also extendinto the radiussed portion 108 and into the vertical portion 106.

As shown, cement 130 is positioned (e.g., pumped) around the casings 120and 122 in an annulus between the casings 120 and 122 and the drillhole104. The cement 130, for example, may secure the casings 120 and 122(and any other casings or liners of the drillhole 104) through thesubterranean layers under the terranean surface 102. In some aspects,the cement 130 may be installed along the entire length of the casings(e.g., casings 120 and 122 and any other casings), or the cement 130could be used along certain portions of the casings if adequate for aparticular drillhole 104. The cement 130 can also provide an additionallayer of confinement for the hazardous material in canisters 126.

The drillhole 104 and associated casings 120 and 122 may be formed withvarious example dimensions and at various example depths (e.g., truevertical depth, or TVD). For instance, a conductor casing (not shown)may extend down to about 120 feet TVD, with a diameter of between about28 in. and 60 in. The surface casing 120 may extend down to about 2500feet TVD, with a diameter of between about 22 in. and 48 in. Anintermediate casing (not shown) between the surface casing 120 andproduction casing 122 may extend down to about 8000 feet TVD, with adiameter of between about 16 in. and 36 in. The production casing 122may extend inclinedly (e.g., to case the horizontal portion 110) with adiameter of between about 11 in. and 22 in. The foregoing dimensions aremerely provided as examples and other dimensions (e.g., diameters, TVDs,lengths) are contemplated by the present disclosure. For example,diameters and TVDs may depend on the particular geological compositionof one or more of the multiple subterranean layers (112, 114, 116, and118), particular drilling techniques, as well as a size, shape, ordesign of a hazardous material canister 126 that contains hazardousmaterial to be deposited in the hazardous waste repository 100. In somealternative examples, the production casing 122 (or other casing in thedrillhole 104) could be circular in cross-section, elliptical incross-section, or some other shape.

As illustrated, the vertical portion 106 of the drillhole 104 extendsthrough subterranean layers 112, 114, and 116, and, in this example,lands in a subterranean layer 118. As discussed above, the surface layer112 may or may not include mobile water. In this example, a mobile waterlayer 114 is below the surface layer 112 (although surface layer 112 mayalso include one or more sources of mobile water or liquid). Forinstance, mobile water layer 114 may include one or more sources ofmobile water, such as freshwater aquifers, salt water or brine, or othersource of mobile water. In this example of hazardous waste repository100, mobile water may be water that moves through a subterranean layerbased on a pressure differential across all or a part of thesubterranean layer. For example, the mobile water layer 114 may be apermeable geologic formation in which water freely moves (e.g., due topressure differences or otherwise) within the layer 114. In someaspects, the mobile water layer 114 may be a primary source ofhuman-consumable water in a particular geographic area. Examples of rockformations of which the mobile water layer 114 may be composed includeporous sandstones and limestones, among other formations.

Other illustrated layers, such as the impermeable layer 116 and thestorage layer 118, may include immobile water. Immobile water, in someaspects, is water (e.g., fresh, salt, brine), that is not fit for humanor animal consumption, or both. Immobile water, in some aspects, may bewater that, by its motion through the layers 116 or 118 (or both),cannot reach the mobile water layer 114, terranean surface 102, or both,within 10,000 years or more (such as to 1,000,000 years).

Below the mobile water layer 114, in this example implementation ofhazardous waste repository 100, is an impermeable layer 116. Theimpermeable layer 116, in this example, may not allow mobile water topass through. Thus, relative to the mobile water layer 114, theimpermeable layer 116 may have low permeability, e.g., on the order ofnanodarcy permeability. Additionally, in this example, the impermeablelayer 116 may be a relatively non-ductile (i.e., brittle) geologicformation. One measure of non-ductility is brittleness, which is theratio of compressive stress to tensile strength. In some examples, thebrittleness of the impermeable layer 116 may be between about 20 MPa and40 MPa.

As shown in this example, the impermeable layer 116 is shallower (e.g.,closer to the terranean surface 102) than the storage layer 118. In thisexample rock formations of which the impermeable layer 116 may becomposed include, for example, certain kinds of sandstone, mudstone,clay, and slate that exhibit permeability and brittleness properties asdescribed above. In alternative examples, the impermeable layer 116 maybe deeper (e.g., further from the terranean surface 102) than thestorage layer 118. In such alternative examples, the impermeable layer116 may be composed of an igneous rock, such as granite.

Below the impermeable layer 116 is the storage layer 118. The storagelayer 118, in this example, may be chosen as the landing for thehorizontal portion 110, which stores the hazardous material, for severalreasons. Relative to the impermeable layer 116 or other layers, thestorage layer 118 may be thick, e.g., between about 100 and 200 feet oftotal vertical thickness. Thickness of the storage layer 118 may allowfor easier landing and directional drilling, thereby allowing thehorizontal portion 110 to be readily emplaced within the storage layer118 during constructions (e.g., drilling). If formed through anapproximate horizontal center of the storage layer 118, the horizontalportion 110 may be surrounded by about 50 to 100 feet of the geologicformation that comprises the storage layer 118. Further, the storagelayer 118 may also have only immobile water, e.g., due to a very lowpermeability of the layer 118 (e.g., on the order of milli- ornanodarcys). In addition, the storage layer 118 may have sufficientductility, such that a brittleness of the rock formation that comprisesthe layer 118 is between about 3 MPa and 10 MPa. Examples of rockformations of which the storage layer 118 may be composed include shale,salt, and anhydrite (among others). Further, in some aspects, hazardousmaterial may be stored below the storage layer, even in a permeableformation such as sandstone or limestone, if the storage layer is ofsufficient geologic properties to isolate the permeable layer from themobile water layer 114.

In some aspects, the layer 118 may have properties suitable for along-term confinement of nuclear waste, and for its isolation from amobile water layer (e.g., aquifers) and a terranean surface. Suchformations may be found relatively deep in the Earth, typically 3000feet or greater, and placed in isolation below any fresh water aquifers.For instance, the appropriate formation may include geologic propertiesthat enhance the long-term (e.g., thousands of years) isolation ofmaterial. Such properties, for instance, have been illustrated throughthe long term storage (e.g., tens of millions of years) of hydrocarbonfluids (e.g., gas, liquid, mixed phase fluid) without escape ofsubstantial fractions of such fluids into surrounding layers (e.g.,mobile water layer).

For example, shale has been shown to hold natural gas for millions ofyears or more, giving it a proven capability for long-term storage ofhazardous material. Example shale formations (e.g., Marcellus, EagleFord, Barnett, and otherwise) has stratification that contains manyredundant sealing layers that have been effective in preventing movementof water, oil, and gas for millions of years, lacks mobile water, andcan be expected (e.g., based on geological considerations) to sealhazardous material (e.g., fluids or solids) for thousands of years afterdeposit. In some aspects, the formation may form a leakage barrier, orbarrier layer to fluid leakage that may be determined, at least in part,by the evidence of the storage capacity of the layer for hydrocarbons orother fluids (e.g., carbon dioxide) for hundreds of years, thousands ofyears, tens of thousands of years, hundreds of thousands of years, oreven millions of years.

Another particular quality of shale that may advantageously lend itselfto hazardous material storage is its clay content, which, in someaspects, provides a measure of ductility greater than that found inother, impermeable rock formations. For example, shale may bestratified, made up of thinly alternating layers of clays (e.g., betweenabout 20-30% clay by volume) and other minerals. Such a composition maymake shale less brittle and, thus less susceptible to fracturing (e.g.,naturally or otherwise) as compared to rock formations in theimpermeable layer (e.g., dolomite or otherwise). For example, rockformations in the impermeable layer may have suitable permeability forthe long term storage of hazardous material, but are too brittle andcommonly are fractured. Thus, such formations may not have sufficientsealing qualities (as evidenced through their geologic properties) forthe long term storage of hazardous material.

In some aspects, the formation of the storage layer 118 and/or theimpermeable layer 116 may form a leakage barrier, or barrier layer tofluid leakage that may be determined, at least in part, by the evidenceof the storage capacity of the layer for hydrocarbons or other fluids(e.g., carbon dioxide) for hundreds of years, thousands of years, tensof thousands of years, hundreds of thousands of years, or even millionsof years. For example, the barrier layer of the storage layer 118 and/orimpermeable layer 116 may be defined by a time constant for leakage ofthe hazardous material more than 10,000 years (such as between about10,000 years and 1,000,000 years) based on such evidence of hydrocarbonor other fluid storage.

The present disclosure contemplates that there may be many other layersbetween or among the illustrated subterranean layers 112, 114, 116, and118. For example, there may be repeating patterns (e.g., vertically), ofone or more of the mobile water layer 114, impermeable layer 116, andstorage layer 118. Further, in some instances, the storage layer 118 maybe directly adjacent (e.g., vertically) the mobile water layer 114,i.e., without an intervening impermeable layer 116. In some examples,all or portions of the radiussed drillhole 108 and the horizontaldrillhole 110 may be formed below the storage layer 118, such that thestorage layer 118 (e.g., shale or other geologic formation withcharacteristics as described herein) is vertically positioned betweenthe horizontal drillhole 110 and the mobile water layer 114.

In this example, the horizontal portion 110 of the drillhole 104includes a storage area in a distal part of the portion 110 into whichhazardous material may be retrievably placed for long-term storage. Forexample, a work string (e.g., tubing, coiled tubing, wireline, orotherwise) or other downhole conveyance (e.g., tractor) may be movedinto the cased drillhole 104 to place one or more (three shown but theremay be more or less) hazardous material canisters 126 into long term,but in some aspects, retrievable, storage in the portion 110.

Each canister 126 may enclose hazardous material (shown as material145). Such hazardous material, in some examples, may be biological orchemical waste or other biological or chemical hazardous material. Insome examples, the hazardous material may include nuclear material, suchas SNF recovered from a nuclear reactor (e.g., commercial power or testreactor) or military nuclear material. Spent nuclear fuel, in the formof nuclear fuel pellets, may be taken from the reactor and not modified.Nuclear fuel pellet are solid, although they can contain and emit avariety of radioactive gases including tritium (13 year half-life),krypton-85 (10.8 year half-life), and carbon dioxide containing C-14(5730 year half-life). Other hazardous material 145 may include, forexample, radioactive liquid, such as radioactive water from a commercialpower (or other) reactor.

In some aspects, the storage layer 118 should be able to contain anyradioactive output (e.g., gases) within the layer 118, even if suchoutput escapes the canisters 126. For example, the storage layer 118 maybe selected based on diffusion times of radioactive output through thelayer 118. For example, a minimum diffusion time of radioactive outputescaping the storage layer 118 may be set at, for example, fifty times ahalf-life for any particular component of the nuclear fuel pellets.Fifty half-lives as a minimum diffusion time would reduce an amount ofradioactive output by a factor of 1×10¹⁵. As another example, setting aminimum diffusion time to thirty half-lives would reduce an amount ofradioactive output by a factor of one billion.

For example, plutonium-239 is often considered a dangerous waste productin SNF because of its long half-life of 24,100 years. For this isotope,50 half-lives would be 1.2 million years. Plutonium-239 has lowsolubility in water, is not volatile, and as a solid, its diffusion timeis exceedingly small (e.g., many millions of years) through a matrix ofthe rock formation that comprises the illustrated storage layer 118(e.g., shale or other formation). The storage layer 118, for examplecomprised of shale, may offer the capability to have such isolationtimes (e.g., millions of years) as shown by the geological history ofcontaining gaseous hydrocarbons (e.g., methane and otherwise) forseveral million years. In contrast, in conventional nuclear materialstorage methods, there was a danger that some plutonium might dissolvein a layer that comprised mobile ground water upon confinement escape.

In some aspects, the drillhole 104 may be formed for the primary purposeof long-term storage of hazardous materials. In alternative aspects, thedrillhole 104 may have been previously formed for the primary purpose ofhydrocarbon production (e.g., oil, gas). For example, storage layer 118may be a hydrocarbon bearing formation from which hydrocarbons wereproduced into the drillhole 104 and to the terranean surface 102. Insome aspects, the storage layer 118 may have been hydraulicallyfractured prior to hydrocarbon production. Further in some aspects, theproduction casing 122 may have been perforated prior to hydraulicfracturing. In such aspects, the production casing 122 may be patched(e.g., cemented) to repair any holes made from the perforating processprior to a deposit operation of hazardous material. In addition, anycracks or openings in the cement between the casing and the drillholecan also be filled at that time.

As shown in this example, a drillhole seal 134 a, such as a plug,packer, or other seal, is positioned in the vertical portion 106 of thedirectional drillhole 104. In some aspects, the drillhole seal 134 a mayprevent or help prevent hazardous waste stored in the canisters 126, orsolids or fluids released by the hazardous waste in the canisters 126,from moving through the vertical portion 106 toward the terraneansurface 102 from the horizontal portion 110. As further shown in thisexample implementation, the drillhole seal 134 a is placed in thevertical drillhole portion 106, while the drillhole seal 134 b is placedin the horizontal drillhole portion 110. Although two drillhole seals134 a and 134 b are shown, more or fewer drillhole seals according tothe present disclosure may be positioned in the hazardous wasterepository 100. Further, in some aspects, both drillhole seals 134 a and134 b (and others is applicable) may be positioned in the verticaldrillhole portion 106. Alternatively, both drillhole seals 134 a and 134b (and others is applicable) may be positioned in the horizontaldrillhole portion 110. In some aspects, one or more drillhole seals(such as 134 a or 134 b) may be positioned in the transition drillholeportion 108. In some aspects, two or more drillhole seals (such as 134 aand 134 b) may be positioned in contact with each other in thedirectional drillhole 104.

In some aspects, one or more of the previously described components ofthe repository 100 may combine to form an engineered barrier of thehazardous waste material repository 100. For example, in some aspects,the engineered barrier is comprised of one, some, or all of thefollowing components: the storage layer 118, the casing 122, thecanister 126, the seal 134, and the hazardous material 145, itself. Insome aspects, one or more of the engineered barrier components may act(or be engineered to act) to: prevent or reduce corrosion in thedrillhole 104, prevent or reduce escape of the hazardous material 145;reduce or prevent thermal degradation of one or more of the othercomponents; and other safety measures to ensure that the hazardousmaterial 145 does not reach the mobile water layer 114 (or surface layer112, including the terranean surface 102).

FIG. 2 is a schematic top view of an example implementation of ahazardous waste repository system 200 formed in one or more subterraneanformations that include one or more faults (205 and 210). For example,the hazardous waste repository system 200 may include one or severalhazardous waste repository 100. In this example, each hazardous wasterepository 100 includes two horizontal drillhole portions 110 (e.g., inwhich hazardous waste, such as nuclear waste, is stored in hazardouswaste canisters 126) that are each coupled to a single verticaldrillhole portion 106. In other examples, there may be more or fewerhazardous waste repositories 100, and each hazardous waste repository100 may have a 1:1 ratio of vertical drillhole portions 106 tohorizontal drillhole portions 110. As shown in this example, one or morefaults 205 and 210 are present. Each fault 205 and 210 may extendthrough a single subterranean formation (e.g., formation 114, orformation 116, or formation 118). Alternatively, in some cases, suchfaults 205 and 210 may extend through multiple subterranean formations(e.g., subterranean formations 116 and 118).

In some aspects, the faults 205 or 210 may be present (e.g., naturally)in a particular subterranean formation (e.g., layer 118) in which thehorizontal drillhole portion 110 that stores the hazardous waste isformed. Due to such naturally occurring faults 205 or 210,conventionally, underground disposal (e.g., in deep, human-unoccupiabledirectional drillholes) of nuclear waste (e.g., spent nuclear fuel orhigh level waste) cannot be done safely in regions in which faults occurdue to likely seismic activity (e.g., earthquakes) associated with thesefaults 205 or 210. Since some nuclear waste is generated in regions thathave large and frequent earthquakes (e.g., nuclear waste from commercialnuclear reactors in California, Taiwan, South Korea, and Japan to name afew), that assumption requires a distant location for disposal. Distantdisposal can create legal issues (some countries are mandated to disposewithin the country) and real or perceived risks from transportation.

In some aspects, the shaking caused by a nearby earthquake is not theprimary danger to the nuclear waste canisters 126 positioned in ahazardous waste repository of the deep directional drillhole 104 formedin the subterranean formation 118 (in this example). The reason is thatsuch accelerations are typically less than 1 g (i.e., less than 980 gal,where a gal is the standard unit for acceleration, equal to 1 cm persecond per second). Such accelerations present threats to surfacestructures, but the nuclear waste canisters 126, in some aspects, may bedesigned to endure much stronger accelerations.

Rock at a depth of the subterranean formation 118, for example, whenaccessed by the directional drillhole 110, typically has a stress tensormeasured by using standard geophysical techniques. Deep horizontalwellbores for hydraulic fracturing and extraction of oil and gas aretypically drilled in the direction of maximum rock stress. For example,horizontal wellbores used to access regions of shale gas and/or shaleoil extend in a direction of maximum horizontal stress in the rock layerthrough which such wellbores are formed. In the locations where an angleof the horizontal drillhole deviates from the direction of maximumhorizontal stress, it does so because for that local rock, the directionof maximum stress changes.

For example, in hydrocarbon recovery from shale, orientation along thedirection of maximum horizontal stress is done because doing so isusually optimum for the process of hydraulic fracturing. In hydraulicfracturing, a series of perforations is made in the casing along itslength in the horizontal section. High pressure water and sand is thenpumped into the casing and out through the holes to fracture the rock.Under this pressure, the rock tends to fracture in the directionperpendicular to the drillhole. The desired outcome is a set of fractureplanes that are perpendicular to the drillhole, so that when many holesare fractured, these fractures will be spread throughout a large volumeof the target shale rock. If the fractures were parallel to thedrillhole, then they would overlap, and a much smaller volume would becovered.

For the underground disposal or storage of radioactive waste (e.g., SNFor high level waste), conventionally, orientation of any wellbore orhuman-occupiable underground repository was selected in an arbitrarydirection without regard for, e.g., maximum horizontal stress of therock layer or direction of faults therethrough. Arbitrary drillholeorientations of the horizontal drillhole portion 110 (e.g., relative tothe vertical drillhole portion 106) may be acceptable, and in some casesuseful, but they may also present a danger in a subterranean formationthat is susceptible to seismic events, such as earthquakes. If a faultslips, that is, if the rock on one side of the fault moves with respectto the other side, and this fault crosses the hazardous waste repositorysection of the directional drillhole 104, then that slippage could breakthe continuity of the drillhole 104, could possible break one or morenuclear waste canisters 126 that is in horizontal drillhole portion 110,and at the same time create a path to the terranean surface 102 thatwould allow fluid, including gases and liquids, to reach the surfacefrom the disposal section in an unacceptably short time.

The hazardous waste repository system 200 shown in FIG. 2 is an exampleimplementation of one or more hazardous waste repositories 100 formed inone or more subterranean formations in regions where earthquakes arefrequent and/or likely to occur in the future. As described with respectto FIG. 1 , the horizontal drillhole portions 110 may be formed in orunder a subterranean formation (e.g., layer 118) that is or includes animpervious or impermeable seal to the transmission of fluid (e.g.,liquid or gas) therethrough. For example, the formation may be shale,salt, clay, or other type of rock.

As shown in this example, in the hazardous waste repository system 200,each of the horizontal drillhole portions 110 of the hazardous wasterepositories 100 are oriented parallel to the particular fault 205 or210 that extends through the same subterranean formation as thedrillhole portions 110. For example, in some aspects, the horizontal (ornearly horizontal) drillholes 110 may achieve a greater safety againstaccidental rupture from earthquake faults 205 or 210 if such drillhole110 are aligned perpendicular to the direction of maximum horizontalstress (that is, parallel to the faults 205 or 210). Thus, the hazardouswaste repository system 200 includes horizontal drillhole portions 110that are parallel to a direction in which a fracture caused by a seismicevent is most likely to occur, either from extension of an existingfault 205 or 210 (which may or may not be known to geologists) or fromthe creation of a new fault during the earthquake or other seismicevent. This orientation may significantly reduce the likelihood that anearthquake-induced fracture will cross the hazardous waste repositoryarea of the deep directional drillhole 104.

In an example operation, a particular subterranean formation (e.g.,storage layer 118) may be determined as being suitable as a hazardouswaste repository. For example, the particular subterranean formation maybe suitable based on, e.g., one or more geologic parameters (e.g.,permeability, ductility, brittleness), one or more test results on aliquid (e.g., brine) found in the subterranean formation, geographiclocation, or other criteria. The suitability of the particularsubterranean formation as a hazardous waste repository may be, forexample, its ability to sealingly store nuclear waste for a long periodof time (e.g., tens of years, hundreds of years, thousands of years,tens of thousands of years).

In a next step, a determination is made of whether one or more faultsextend through the particular subterranean formation and, if so, wheresuch fault (or faults) extend. For example, in some aspects, faults maybe located (and known) from seismic records. In some aspects, faults maybe identified from seismic reflection surveys (showing discontinuitiesin competent strata) or through geologic mapping. In some aspects,indirect methods, e.g., detection of methane leaks, might detect faultsat a terranean surface, such as if the fault is hydraulicallyconducting. In some aspects, electromagnetic (EM) surveys may alsodiagnose fault locations.

As further examples, faults may be known or determined since many faultsoccur in conjugate planes. For example, while the Hayward fault zone inCalifornia is a known fault, there are also faults at an acute (e.g.,not quite 90 degrees) angle to the Hayward fault that can also createearthquakes. Thus, in some aspects, deep directional drillholes may beformed that are parallel to known (and the riskiest) faults while stillintersecting other (less risky) faults. As another example, Taiwan is atan example of a collision margin (e.g., convergent margin where tectonicplates are moving toward each other). Thus, in this area, most of thefaults are thrust and reverse with fault planes oriented in thecollision direction.

Relatedly, a stress tensor of the rock at depth can be determined usingthe geophysical techniques that are used in the oil and gas technology.However, even before drilling, it can be estimated by mapping of nearbyfaults. In a region that has a stress tensor that varies only slowlyover location, the earthquake faults will be parallel, and thus mappinggives the desired direction.

In a next step, a directional drillhole (e.g., a vertical portion,radius portion, and horizontal portion) may be formed and orientedparallel to the determined one or more faults without any intersectionbetween the drillhole and the fault(s). The hazardous waste repositoryis then formed in, e.g., the horizontal portion of the directionaldrillhole and hazardous waste (e.g., radioactive waste) is emplacedtherein. In some aspects, in an area that has nearby faults, an optimumlocation for a deep directional drillhole may be between two parallelfaults and originated in the same direction as the faults. Thus, neitherof these faults intersects the hazardous waste repository area of a deepdirectional drillhole, nor would such faults intersect the hazardouswaste repository area if an earthquake lengthened the faults. In someaspects, the existence of these faults provides a relief for stress thatcould build in the rock; stress that, in the absence of these existingfaults, might create a new fault.

FIG. 3 is a schematic illustration of an example implementation of adrillhole seal 300 for a hazardous waste repository, such as hazardouswaste repository 100 shown in FIG. 1 . In some aspects, the drillholeseal 300 may be used as one or both of drillhole seals 134 a or 134 b inthe repository 100. As shown in the example implementation of FIG. 1 ,for example, drillhole seal 134 a is positioned in the verticaldrillhole portion 106. In some aspects, when a drillhole (or wellbore),such as drillhole portion 106, is created, there is typically a“disturbed zone” that circumscribes the created drillhole, oftenextending into the rock layer a distance roughly equal to the radius ofthe drillhole. This disturbed zone provides a potential pathway for thewaste (such as hazardous waste 145 if leaked from canisters 126) to becarried to the terranean surface 102 by, e.g., flowing water and brine.Another possible leakage path is in the space 302 (e.g., annulus 302)between a casing (e.g., the casing 120) and the rock formation (e.g.,formations 112 through 118 and others) through which the drillhole 104extends. For some drillholes, the annulus 302 is filled with cement. Thecapability to prevent leakage in this location is related, in someaspects, to the lifetime of the cement.

The drillhole seal 300, as shown in FIG. 3 , is placed in the verticaldrillhole portion 106 of the deep directional drillhole 104 (e.g., ahuman-unoccupiable drillhole) that includes or is part of the hazardouswaste repository 100. In some aspects, the seal 300 (also called a plug)may utilize a geologic formation property called “creep,” which mayadvantageously be utilized to seal the vertical drillhole portion 106(in this example). Creep occurs, for example, in shale, in clay, insalt, and other rocks. Creep, generally, is a slow flow of the rock thattends to fill cracks and other discontinuities. In the exampleimplementation shown in FIG. 3 , the seal 300 includes one or more rockportions made of particular rocks (or a particular rock) that exhibitscreep.

As shown in FIG. 3 (and also with reference to components shown in FIG.1 ), the drillhole seal 300 includes a frame or housing 304 that atleast partially encloses a rock (or other natural) material 306. Therock material 306, in some aspects, is shale, or clay, or salt, or otherrock material that exhibits creep. In some aspects, the frame or housing304 is at least partially open to the annulus 302, the subterraneanformation 116, or both. Thus, in some aspects, as the rock material 306exhibits creep, the material 306 may move to fill in spaces between theframe 304 and, e.g., the formation 116 (e.g., fill in all or part of adisturbed zone around the vertical drillhole portion 106).

In this example, a diameter, d, of the drillhole seal 300 is larger thana diameter, D, of the casing 120. Thus, in this example, a milledportion 308 of the casing 120 (and cement 130 and perhaps the formation116) may be removed prior to installation of the drillhole seal 300 inthe vertical drillhole portion 106. In such aspects, an expandableportion of the drillhole seal 300 (not shown), such as a packer-typedevice, may be expanded to adjust the drillhole seal 300 into the milledportion 308. In alternative aspects, the diameter, d, of the drillholeseal 300 may be less than the diameter, D, of the casing 120. In suchaspects as well, an expandable portion of the drillhole seal 300 (notshown), such as a packer-type device, may be expanded to adjust thedrillhole seal 300 to contact the casing 120. In alternative aspects,the diameter, d, of the drillhole seal 300 may be equal to orapproximately equal to the diameter, D, of the casing 120.

The example implementation of the drillhole seal 300 can be used at arock layer that is similar in composition to the rock material 306(e.g., shale, clay, salt, or other rock), but it can also be used atother layers (e.g., rock layers made of limestone or basalt) that aredissimilar to the rock material 306. In some aspects, the rock material306 could be obtained from the drillhole 104 itself; if more material306 is needed (e.g., for multiple drillhole seals 300), then suchmaterial 306 could be brought in from another location.

In some aspects, salt can also be used for the rock material 306 in theseal 300. For example, salt has the property that local water candissolve it, and that could help move it into the damaged zone (e.g.,adjacent the milled portion 308) to help seal the damaged zone thatcircumscribes the drillhole portion 106. However, in some aspects, caremust be taken to assure that any salt not be vulnerable to dissolutionby deep brines. An indicator of safety against dissolution would be ifthose brines are already saturated with salt.

In another example implementation, the rock material 306 may becomprised of multiple types of rock, at least one of which exhibitingthe property of creep. Some of these types of rock material 306 maymatch the rock layer, e.g., in the subterranean formation 116, thusmaking the hole filled be similar at depth to the pre-existing rock. Therock material 306 of the seal 300 may not be identical to thesubterranean formation 116, since the rock in the layer 116 will besolid, although perhaps cracked in the disturbed zone, but the rockmaterial 306 in the seal 300 may consist of smaller pieces in order tobe put in place. In some aspects, matching the rock type at depth mayprovide a more continuous seal that would otherwise be available with aconventional hydrocarbon operations seal (e.g., bridge plug, packer, orotherwise). In addition, in some aspects, the drillhole seal 300 mayinclude other sealing materials, such as, for example, cement,bentonite, or other sealing material.

Although described as positioned in the vertical drillhole portion 106,the drillhole seal 300 can be used with drillholes with any orientation(or formed for other purposes). A drillhole seal 300 in accordance withthis disclosure could be used, for example, to seal conventional oil andgas wells.

In an example operation, once the waste canisters have been emplaced inthe directional drillhole 104, the milled portion 308 (e.g., of thecasing 120 and cement 130 and possibly part of the layer 116) isoptionally formed (e.g., with a reaming tool). For example, the portionof the casing 120 may be removed (e.g., milled out or otherwise cutaway) to improve a secure seal between the seal 300 and the surroundingrock formation 116. In some aspects, a seal-to-casing seal may not be assecure. For example, without removing the portion of the casing 120,there is a possibility that a pathway will exist in the annulus 302 (orotherwise radially outside of the casing 120) that could convey water orbrine, perhaps containing hazardous waste 145 to the terranean surface102 or near surface (e.g., to a source of mobile water).

In some aspects, only a portion of the casing 120 in the verticaldrillhole 106 is removed, as it may not be necessary to remove all ofthe casing 120. For example, in some aspects, the intent is to seal thevertical drillhole 106 at several locations, and not necessarily at alldepths. By removing the portion of the casing 120, a disc-shaped portionwith an outer diameter greater than the diameter, D, of the casing 120(and possibly the vertical drillhole portion 106) is formed at aparticular depth in the vertical drillhole portion 106.

In some aspects, the horizontal disposal repository drillhole sitsunderneath one or more layers of clay-rich shale, a rock that hasappropriate creep properties. The vertical drillhole portion 106 maypenetrate this layer. The shale (e.g., a “cap” layer) may also beself-healing and reduce fractures and other pathways that could allowgases and liquids to move quickly through the layer.

Once the portion of the casing 120 is removed, drillhole seal 300 isinserted into the vertical portion 106 of the drillhole 104 to the depthat which the casing portion is removed. As the disc-shaped volumecreated by the removal of the casing 120 is filled by the drillhole seal300, the pressure of rock at shallower depths presses against the rocklayer 116 over a short period of time (days to years), thereby causingcreep that “heals” (e.g., fills in) small cracks and discontinuities inthe disc-shaped volume (e.g., the milled portion 308). As described, thedownhole seal 300 may include the frame 304 or other structure (e.g.,made of a corrosion resistant material) that holds or at least partiallyencloses the creep material 306.

Such filling material 306 is not standard for sealing holes in the oiland gas industry since a quicker seal is typically desired. For thatreason, cement is frequently a component. However, implementations ofthe drillhole seal 300 described here provides an engineered barrier forstrong isolation and protection for thousands of years by takingadvantage of the process of creep to provide a better long-term seal fora hazardous waste repository.

The present disclosure also contemplates implementations of systems andmethods to seal a portion (vertical portion 106, horizontal portion 110,or both) of the directional drillhole 104 that stores hazardous (e.g.,nuclear) waste that include multiple drillhole seals 300 positioned inthe directional drillhole 104. For example, once the waste canisters 126have been emplaced in the directional drillhole 104, portions of thecasing 120 at two or more depths are removed.

In some aspects, each drillhole seal 300 includes rock material 306 thatmatches the natural geologic formation at the particular depth of eachseal 300, and which, from the pressure of rock above, forms a good sealwith that geologic formation. In some aspects, there may be a respectivedrillhole seal 300 positioned in the vertical portion 106 at eachdifferent geologic formation (e.g., subterranean formations 112, 114,116, 118) between the terranean surface 102 and a formation in which ahazardous waste repository is located (e.g., subterranean formation118). Alternatively, there may be a respective drillhole seal 300positioned in the vertical portion 106 at less than all of the geologicformations between the terranean surface 102 and the formation 118 inwhich a hazardous waste repository is located.

To install each drillhole seal 300, in this example, a portion of thecasing 120 is removed as previously described. A particular drillholeseal 300 that includes a material 306 that has good creep propertiesand/or matches a geologic formation at a desired set-depth of the seal300 is inserted into the vertical portion 106 of the drillhole 104 tothe depth at which the casing portion is removed. As described, in thisexample, each seal 300 includes rock material 306 that matches theformation at a particular depth at which the seal 300 is to be set. Tofacilitate creep, the rock material 306 may be divided into small piecesprior to being formed into the seal 300. For example, if the spacebetween pieces is less than 0.1 mm, then the creep time to fill the gapsis shorter than if there are gaps of several millimeters (or greaterdistances).

In an example implementation of this method, the rock material 306 usedto fill the annulus 302 at the depth would be rock that was obtainedfrom that layer (e.g., subterranean formation 112, 114, 116, or 118)when the drillhole 104 was originally formed. Alternatively, the rockmaterial 306 could be rock obtained from another drillhole, or it couldbe rock obtained from a location in which the same geologic formationcomes closer to or reaches the terranean surface 102. Thus, the rockmaterial 306 used to form the seal 300 may have as good of a match withthe geologic formation as possible to assure that over time there willbe little to no unconformity between the fill and the undrilled rock.

In some aspects, there is not a seal 300 set to match every formation inthe case of “layer cake” geology, i.e. geology that consists of manylayers of different kinds of rock. In some aspects, there may be onlyone seal 300 that is set at a layer that will provide a seal againstleakage.

FIGS. 4A-4B are schematic illustrations of an example implementation ofa nuclear waste dry cask 400 that encloses one or more nuclear wastecanisters 420 for a hazardous waste repository according to the presentdisclosure. As shown, FIG. 4A shows a vertical cross-section of thenuclear waste dry cask 400 in which multiple nuclear waste canisters 420are enclosed. FIG. 4B shows a radial cross-section taken from FIG. 4A.Generally, the nuclear waste dry cask 400 may enclose and store, for atransient amount of time, nuclear waste, such as SNF or high levelwaste. For instance, after spending several years in a cooling pool,nuclear waste in the form of SNF assemblies may be moved to “dry cask”storage. Currently, about one-third of the SNF inventory in the UnitedStates is in such storage, but that fraction is expected to growrapidly. A conventional SNF assemblies is typically a rectangular solidin shape, between 8 to 12 inches wide (diagonal dimension of squarecross section), and 14 feet long. In some aspects, thirty-six or more ofthe SNF assemblies are placed in a conventional canister (with adiameter of about 5 feet). The canister is filled with helium gas (todistribute heat generated by the assemblies), sealed, and placed insidea conventional concrete cask. A conventional cask may have walls thatare typically 2 feet thick, which provide radiation (e.g., gamma ray)shielding for people in the vicinity of the cask. Air is circulatedaround the large canister (within the dry cask) to provide cooling.

Conventional dry cask storage is designed (and licensed) for temporarystorage. For permanent storage, the top concrete lid is removed, thecanister weld broken, and the SNF assemblies lifted out and placed indisposal canisters. These steps must be done either under water or in a“hot cell” (e.g., a room certified for handing nuclear material) sincethe fuel assemblies can emit gases and other radioactive material.

Example implementations of the nuclear waste dry cask 400 facilitatestransfer of one or more nuclear waste canisters 420 (that may becircular, square, rectangular, or other shape in cross-section) that arestored in the nuclear waste dry cask 400 into permanent disposal (e.g.,for hundreds if not thousands of years) in a deep, human-unoccupiabledirectional drillhole (such as drillhole 104 shown in the hazardouswaste repository 100 in FIG. 1 ). In some aspects, nuclear waste 426,shown as an example in one of the canisters 426, is SNF in one or moreSNF assembles that are formed from multiple SNF rods 428. As shown inFIGS. 4A-4B, nuclear waste 426 may represent a single SNF assembly withmultiple rods 428 (however, other example implementations of the nuclearwaste canister 426 may store multiple SNF assembles or even just aportion of a single SNF assembly). Other implementations of the nuclearwaste dry cask 400 may store high level nuclear waste.

In this example implementation, each nuclear waste canister 420 includesa housing 421 to which a lid 422 and bottom 424 are sealed (e.g.,subsequent to emplacement of the nuclear waste 426). In some aspects,the example implementations include the loading of SNF assemblies 426 inindividual nuclear waste canisters 420 (i.e., each SNF assembly 426 isloaded into a single canister 420 and each canister 420 is sized toenclose a single SNF assembly 426). In some aspects, the nuclear wastecanister 420 may include radiation shielding (e.g., for gamma ray orX-ray radiation) around (or as part of) the circumferential housing 421of the canister 420 (e.g., from and between lid 422 to bottom 424) butnot at (or as part of) the lid 422 or bottom 424 of the canister 420.Multiple canisters 420 may then be loaded into the nuclear waste drycask 400. Thus, in this example implementation of the nuclear waste drycask 400, a single large canister that holds many (e.g., 36) SNFassemblies is not placed into the nuclear waste dry cask 400 butinstead, multiple SNF canisters 420 are loaded into the nuclear wastedry cask 400. Once loaded, individual SNF canisters 420 can later beremoved without the need for a hot cell or a cooling pool. The canisters420 are sealed and prevent leakage of radioactive material (such as theSNF assemblies 426).

As shown in FIG. 4A, the nuclear waste dry cask 400 includes a top 404and a bottom 406 that connect with a housing 402 to define an innervolume 408 into which the nuclear waste canisters 420 may be emplaced.One or more cooling fluid flow paths 410 may be defined in the volume408 through which a cooling medium (e.g., airflow, liquid coolant, orotherwise) is circulated to remove heat from the nuclear waste canisters420. Further, one or both of the top 404 or bottom 406 may be moveableto expose the volume 408 to an environment external to the nuclear wastedry cask 400. In some aspects, one or both of the top 404 or bottom 406is moveable without fully detaching the top 404 or bottom 406 from thehousing 402, such as through a hinge between the top 404 or bottom 406and the housing 402. Alternatively, one or both of the top 404 or bottom406 may radially pivot (e.g., in an arc) about a pivot connection withthe housing 402 to swing and expose the volume 408 to the environment.

In this example implementation, each of the top 404, the bottom 406, andthe housing 402 of the nuclear waste dry cask 400 includes or iscomprised of radiation (e.g., gamma ray) shielding sufficient toprotect, e.g., humans in an area near the cask 400, from such radiation.In some aspects, the top 404, bottom 406, and housing 402 include or aremade of concrete of a sufficient thickness to provide such radiationshielding. Alternatively, the top 404, bottom 406, and housing 402include or are made of another shielding material, such as tungsten, ofa sufficient thickness to provide such radiation shielding. In someaspects, a thickness of tungsten (as a non-cementitious materialexample) sufficient for radiation shielding is less than, and perhapsorders of magnitude less than, a thickness of concrete sufficient forradiation shielding.

As noted, in some aspects, each SNF canister 420 does not provide orexcludes radiation shielding from the gamma rays except at the lid 422and bottom 424. When the SNF canisters 420 are removed from the nuclearwaste dry cask 400, the canisters 420, in some aspects, be inserted intoa smaller concrete shield or lowered directly into a vertical entranceof the deep directional drillhole 104. For example, when inserting thenuclear waste canisters 420 stored in the nuclear waste dry cask 400into the vertical entrance, the dry cask 400 may be placed above thevertical opening of the drillhole 104 (e.g., of the vertical portion106). The bottom 406 of the dry cask 400 may be removed or moved (e.g.,slid out to a side of the nuclear waste dry cask 400) to expose theinner volume 408 of the cask 400 in which the SNF canisters 420 areenclosed. The position of the dry cask 400 may be adjusted until aparticular one of the SNF canisters 420 is in position above thevertical entrance of the disposal drillhole 104. The canister 420 isthen lowered through the vertical entrance and ultimately to a hazardouswaste repository in a horizontal drillhole portion 110 of the deepdirectional drillhole 104, e.g., for permanent storage. This process maybe repeated, e.g., for each SNF canister 420 stored in the nuclear wastedry cask 400.

In an alternative aspect, the top 404 of the nuclear waste cask 400 isremoved (e.g., completely, slid away, or rotated away) to expose theinner volume 408, and the canisters 420 are raised into a smallertransfer cask. A transfer cask, in some aspects, may be a smallerversion of the nuclear waste dry cask 400 and is designed to hold,typically, one SNF canister 420 (although it could hold two or more, butless than the nuclear waste dry cask 400). The transfer cask is muchsmaller than the nuclear waste dry cask 400 and may contain noparticular cooling system since the SNF canister 420 may be in thistransfer cask only for a short period of time (e.g., relative to thenuclear waste dry cask 400). The transfer cask is then moved to thedisposal drillhole 104, and the SNF canister 420 lowered into thevertical opening as described.

Thus, in some aspects, once the inner volume 408 is exposed, one or moreSNF canisters 420 may be removed (e.g., lowered) from the volume 408into a vertical entrance of a deep directional drillhole 104 (oralternatively into a transfer cask). In some aspects, no additionalgamma ray shielding (besides that of the nuclear waste dry cask 400 andthe individual SNF canisters 420, as described) is required or used toplace the SNF canisters 420 into transfer casks or directly into thedirectional drillhole 104. Implementations of the nuclear waste dry cask400 according to the present disclosure may, therefore, significantlysimplify the process of transfer from dry cask temporary storage topermanent disposal in deep directional drillholes.

FIGS. 5A-5B are schematic illustrations of an example implementation ofa power generator system 500 for a hazardous waste repository in adirectional drillhole. In some aspects (and with reference to certaincomponents described and shown in FIG. 1 ), when hazardous waste, suchas nuclear waste (e.g., SNF or high level waste or both), is disposedunderground, such as in a human-unoccupiable deep directional drillhole(e.g., in drillhole 104), there may be instruments (as all or part of ahazardous waste repository monitoring system) placed near on in contactwith one or more hazardous waste canisters (e.g., canisters 126) thatstore the hazardous waste (e.g., hazardous waste 145). In some aspects,the instruments (e.g., to measure radiation, temperature, pressure, andother environmental conditions in or surrounding the drillhole 104) maybe able to communicate to the terranean surface 102. Such communicationcan facilitate “performance confirmation” of the hazardous wasterepository 100, e.g., as possibly required by a regulatory agency.

In some aspects, all or a part of a hazardous waste repositorymonitoring system (e.g., instruments, sensors, controllers, orotherwise) utilizes electrical power. Conventionally, such power couldbe supplied by a cable that extends within the directional drillhole 104to the surface 102, but that cable then creates a pathway along whichhazardous waste could escape (e.g., through a mobile liquid). Aconventional alternative to wired power could be to include a battery atdepth, but batteries have limited lifetimes.

Example implementations of the power generator system 500 generateselectrical power in the hazardous waste repository of a deep directionaldrillhole 104, e.g., to power one or more instruments that monitorhazardous waste stored in the repository. In some aspects, the hazardouswaste is radioactive waste, such as SNF or high level waste. In someaspects. the power generator system 500 utilizes heat generated by thestored radioactive waste to generate electrical power.

As shown in FIG. 5A (and with reference to certain components shown inFIG. 1 ), nuclear waste canisters 126 that enclose radioactive waste 145(e.g., SNF assemblies) are emplaced in a hazardous waste repository inthe directional drillhole 104. For example, in some aspects, a singlenuclear waste canister 126 that stores nuclear waste in the directionaldrillhole 104 (and more specifically, the horizontal drillhole portion110) produces heat energy at a rate of several hundred watts. As thatenergy is conducted away, e.g., through any filling within the drillholeportion 110, through casing 120 (if casing is used) and into the rock ofthe subterranean formation 118, the heat creates a temperaturedifference that can be exploited to obtain electric (or mechanical)power.

In an example implementation, a nuclear waste canister 126 may enclose asingle spent nuclear fuel assembly (represented as 145 in FIG. 5A).There may be several nuclear waste canisters 126 placed end-to-endwithin the hazardous waste repository of the directional drillholeportion 110. A temperature profile of such a configuration is shown inFIG. 5C, as a chart 550 of temperature at a radial distance from centerof the canisters 126 along a cased drillhole axis (e.g., of thedrillhole portion 110). In chart 550, the x-axis represents a distance(in meters) along the horizontal drillhole portion 110 in which thecanisters 126 are emplaced. The y-axis represents a radial distance (inmeters) from a centerline axis of the canisters 126.

This configuration assumes a 100 W rate of power generation per canister126 and a four foot spacing between canisters 126. In this example, thetemperature difference between the end of the canisters 126 and thecenter of the four foot (cooler) gap between the canisters is about 20°C. This temperature difference, in some aspects, can be used to generatepower by the power generator system 500.

In the example implementation of power generator system 500,thermoelectric power is generated from heat that is output from thenuclear waste 145. For instance, the power generator system 500 may useor include a radioisotope thermal generators (RTG). As shown in FIG. 5A,the power generator system 500 is positioned in the directionaldrillhole 104 (e.g., in the horizontal drillhole portion 110) in anannular space between, e.g., the four foot gap between, adjacent nuclearwaste canisters 126. The example power generator system 500 includes atleast one flat sheet thermoelectric generator 506 positioned betweenheat transfer conductors 504 and heat transfer conductors 502. In thisexample, the heat transfer conductors 504 may be heat source conductors504, as they are positioned closer to a heat source of the closestnuclear waste canister 126. The heat transfer conductors 502 may be heatsink conductors 502, as they are positioned further (relative to theconductors 504) from the heat source of the closest nuclear wastecanister 126.

An example implementation of the thermoelectric generator 506 is shownin FIG. 5B. As shown, the thermoelectric generator 506 includes plates507 and 509 (e.g., ceramic plates) that provide thermal (e.g.,conductive) contact with the respective heat sink conductors 502 and theheat source conductors 504. In some aspects, the plates 507 and 509 maybe the conductors 502 and 504, respectively. Mounted between the plates507 and 509 are n- and p-type semiconductor materials 511 that are incontact with the plates 507 and 509 through conductive members 513.Poles 514 are electrically connected to the thermoelectric generator 506to provide current, I, based on operation of the generator 506.

In some examples, the power generator system 500 may include or beplaced in a container to hold the illustrated components. As shown,springs 508 are included to bias or urge the heat source conductors 504and heat sink conductors 502 against the casing 120. However, the powergenerator system 500 could also be put inside a container that servesanother purpose. For example, the power generator system 500 could beplaced inside a device that measures temperature and pressure and theradioactivity of the environment, and which records such data orbroadcasts the data to a distant recorder (e.g., on the terraneansurface 102).

In FIG. 5A, the flat sheet of the thermoelectric generator 506 is shownperpendicular to the illustration, but the generator 506 could be anyorientation. As shown, a region to the right of the heat sink conductor502 is empty. However, in some aspects, another set of thermoelectricconductors 506 can be placed in that region as well. For example, FIG.5A shows the power generator system 500 filling the space 516 betweentwo nuclear waste canisters 126 in the horizontal drillhole portion 110.In some aspects, however, depending on the desired amount of generatedpower, fewer thermoelectric generators 506 can be used, although longerheat conductors 502 and 504 might then be used to bring in heat from thehottest regions 518 of the drillhole portion 110.

As further shown in FIG. 5A, radiation (e.g., gamma ray) shields 512 maybe placed on ends of the nuclear waste canisters 126. A furtherradiation shield 510 may also be positioned near or in contact with thepower generator system 500 to, e.g., reduce the exposure of thethermoelectric generators 506 to gamma rays from the nuclear waste 145.In some aspects, additional shielding could be added to the powergenerator system 500. In some aspects, radiation shielding is a tungstenshield, since tungsten is a gamma ray absorber and has good long-termanti-corrosion properties. Other gamma ray absorption materials may beused.

In some aspects, the power generator system 500 is made of radiationresistant materials. Example implementations that use radiationresistant materials may not generate as much power (e.g., current) as adesign that uses conventional materials. Such generators might use, forexample, metals with different thermoelectric properties in contact witheach other, rather than using semiconductors.

In operation, the example implementation of the power generator system500 has no moving parts other than the springs 508, which urge the heattransfer conductors 502 and 504 against the casing 120. Duringoperation, only electrons move. The heat source conductor 504 conductsheat along a gap between the nuclear waste canisters 126 to a side of athermoelectric generator 506. Another side of the thermoelectricgenerator 506 is in thermal contact (e.g., conductive) with a heat sink,e.g., a material in the drillhole portion 110 that is cooler than thecanister 126, through the heat sink conductor 502. For example, the heatsink material may be fluid 127 or other filler material 127 within thedrillhole portion 110, or the material may be an inner surface of thedrillhole casing 120 (e.g., a carbon steel casing). Based on atemperature difference between the conductors 502 and 504 and across thethermoelectric generator 506, an electrical current is generated by thethermoelectric generator 506, which can be provided to one or morecomponents or systems in the hazardous waste repository that requireelectrical power.

In some aspects, the heat source conductor 504 may be attached to thenuclear waste canister 126 or other hot surface, such as the drillholefiller or casing 120 in a hot region near a canister 126. In someaspects, one or both of the heat source conductor 504 or heat sinkconductor 502 may be a rod made of metal or some other conductivematerial such as glass. Alternatively, one or both of the heat sourceconductor 504 or heat sink conductor 502 may be a heat pipe or otherheat transfer device such as a tube containing a gas such as helium.

In some aspects, the power generator system 500 may provide other formsof power besides electrical power. For instance, based on the describedtemperature difference between the heat source and the heat sink, adifferential pressure pump may be implemented based on a densitydifference at two ends of a tube caused by the temperature difference.If the tube were then closed in the middle, the density difference wouldbecome a pressure difference, and this pressure difference could be usedto drive a generator or to send a signal directly to the surface usingan acoustic wave. As another example, a power generator system maygenerate power directly from the gamma rays generated by the nuclearwaste rather than from a temperature difference.

FIG. 5D is a flowchart that illustrates an example process 580 thatincludes generating electric power with the power generator system 500of FIG. 5A. Process 580 may begin at step 581, which includes placingone or more nuclear waste canisters and a power generator system (e.g.,power generator system 500) in a hazardous waste repository of adirectional drillhole. For example, one or more nuclear waste canistersthat enclose radioactive, or nuclear, waste may be emplaced in thehorizontal drillhole portion of the directional drillhole. In someaspects, all or a portion of the horizontal drillhole portion comprisesa hazardous waste repository. The radioactive waste generates heat andradiation (e.g., gamma rays). In some aspects, the generated heat istransferred from the waste to the canister and, in some cases, anannulus of the drillhole portion. In an example embodiment, the emplacedpower generator system is positioned in a space between adjacent nuclearwaste canisters in the horizontal drillhole portion.

Process 580 may continue at step 582, which includes urging heattransfer members of the power generator system into thermal contact witha heat source and a heat sink in the hazardous waste repository. Forexample, the power generator system may include a first heat transfermember that is urged (e.g., with springs or another biasing member) intothermal contact (e.g., conductive thermal contact, or convective thermalcontact, or both) with a heat source. The heat source may be, forexample, one or more nuclear waste canisters, a casing portion heated byone or more nuclear waste canisters, a drillhole backfill materialheated by one or more nuclear waste canisters, or a combination thereof.The power generator system may include a second heat transfer memberthat is urged (e.g., with springs or another biasing member) intothermal contact (e.g., conductive thermal contact, or convective thermalcontact, or both) with a heat sink. The heat sink may be, for example, afluid in the drillhole, a casing portion unheated by one or more nuclearwaste canisters, a drillhole backfill material unheated by one or morenuclear waste canisters, or a combination thereof

Process 580 may continue at step 583, which includes thermallycontacting, with a heat transfer member, the heat source in thehazardous waste repository. For example, the first heat transfer memberis placed into thermal contact (and in some aspects, physical contact)with the heat source so that a temperature of the first heat transfermember is adjusted to at or near a temperature of the heat source.

Process 580 may continue at step 584, which includes thermallycontacting, with another heat transfer member, the heat sink in thehazardous waste repository. For example, the second heat transfer memberis placed into thermal contact (and in some aspects, physical contact)with the heat sink so that a temperature of the second heat transfermember is adjusted to at or near a temperature of the heat sink (whichis less than the heat source).

Process 580 may continue at step 585, which includes generating electricpower with a thermoelectric generator thermally coupled to the heattransfer members based on a temperature difference between the heatsource and the heat sink. For example, the thermoelectric generator isthermally coupled to the heat transfer members such that the temperaturedifference between the heat source and heat sink is translated across,e.g., semiconductor material of the thermoelectric generator. Thesemiconductor material generates an electric current based on thetemperature difference.

Process 580 may continue at step 586, which includes supplying thegenerated electric power to a portion of the hazardous waste repository.For example, in some aspects, the generated electrical power may besupplied to one or more components of a hazardous waste repositorymonitoring system, such as radiation sensors, temperature sensors,liquid sensors, or control systems (e.g., microprocessor based systems)communicably coupled to such sensors. In some aspects, such sensors orcontrol systems (e.g., as described in U.S. patent application Ser. No.16/430,005, incorporated by reference herein) may be located in thehorizontal drillhole portion, another drillhole portion of thedirectional drillhole, or another directional or vertical drillholeformed in or adjacent the subterranean formation in which the hazardouswaste repository is located. In some aspects, such sensors or controlsystems may be located at or near the terranean surface and electricalpower is supplied from the power generator system toward the surface.

While this specification contains many specific implementation details,these should not be construed as limitations on the scope of anyinventions or of what may be claimed, but rather as descriptions offeatures specific to particular implementations of particularinventions. Certain features that are described in this specification inthe context of separate implementations can also be implemented incombination in a single implementation. Conversely, various featuresthat are described in the context of a single implementation can also beimplemented in multiple implementations separately or in any suitablesubcombination. Moreover, although features may be described above asacting in certain combinations and even initially claimed as such, oneor more features from a claimed combination can in some cases be excisedfrom the combination, and the claimed combination may be directed to asubcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. In certain circumstances, multitasking and parallel processingmay be advantageous. Moreover, the separation of various systemcomponents in the implementations described above should not beunderstood as requiring such separation in all implementations, and itshould be understood that the described program components and systemscan generally be integrated together in a single software product orpackaged into multiple software products.

A first example implementation according to the present disclosureincludes a nuclear waste dry cask that includes a housing that at leastpartially defines an inner volume sized to enclose a plurality ofnuclear waste canisters. Each nuclear waste canister is sized to store aportion of nuclear waste. The housing includes radiation shielding. Thecask further includes a lid sized to enclose a top opening of the innervolume and comprising radiation shielding; and a bottom sized to enclosea bottom opening of the inner volume and comprising radiation shielding.

In an aspect combinable with the first example implementation, theportion of nuclear waste comprises a spent nuclear fuel (SNF) assembly.

In another aspect combinable with any of the previous aspects of thefirst example implementation, each nuclear waste canister is sized tostore a single SNF assembly.

In another aspect combinable with any of the previous aspects of thefirst example implementation, the radiation shielding comprises amaterial that absorbs gamma rays or prevents gamma rays from passingtherethrough.

In another aspect combinable with any of the previous aspects of thefirst example implementation, the bottom is configured to move to exposethe bottom opening of the inner volume.

In another aspect combinable with any of the previous aspects of thefirst example implementation, the bottom is slideable to expose thebottom opening of the inner volume.

In another aspect combinable with any of the previous aspects of thefirst example implementation, each SNF canister comprises radiationshielding only on a top surface and a bottom surface of the SNFcanister.

In another aspect combinable with any of the previous aspects of thefirst example implementation, the radiation shielding comprisesconcrete.

In another aspect combinable with any of the previous aspects of thefirst example implementation, the lid is configured to move to exposethe top opening of the inner volume.

In another aspect combinable with any of the previous aspects of thefirst example implementation, the plurality of nuclear waste canisterscomprise at least fifteen nuclear waste canisters.

A second example implementation according to the present disclosureincludes a method for storing nuclear waste that includes placing aplurality of nuclear waste canisters in a nuclear waste dry cask thatcomprises a housing that at least partially defines an inner volumesized to enclose the plurality of nuclear waste canisters. Each nuclearwaste canister is sized to store a portion of nuclear waste. The housingincludes radiation shielding. The method further includes enclosing atop opening of the inner volume with a lid that comprises radiationshielding; and enclosing a bottom opening of the inner volume with abottom that comprises radiation shielding.

In an aspect combinable with the second example implementation, theportion of nuclear waste comprises a spent nuclear fuel (SNF) assembly.

In another aspect combinable with any of the previous aspects of thesecond example implementation, each nuclear waste canister is sized tostore a single SNF assembly.

In another aspect combinable with any of the previous aspects of thesecond example implementation, the radiation shielding comprises amaterial that absorbs gamma rays or prevents gamma rays from passingtherethrough.

Another aspect combinable with any of the previous aspects of the secondexample implementation further includes moving the nuclear waste drycask over a vertical entrance of a directional drillhole.

Another aspect combinable with any of the previous aspects of the secondexample implementation further includes moving the bottom to expose thebottom opening of the inner volume to the vertical entrance.

In another aspect combinable with any of the previous aspects of thesecond example implementation, moving the bottom comprises sliding thebottom to expose the bottom opening of the inner volume to the verticalentrance.

Another aspect combinable with any of the previous aspects of the secondexample implementation further includes moving at least one of thenuclear waste canisters out of the inner volume, through the bottomopening, and into the vertical entrance.

Another aspect combinable with any of the previous aspects of the secondexample implementation further includes moving all of the nuclear wastecanisters out of the inner volume, through the bottom opening, and intothe vertical entrance.

Another aspect combinable with any of the previous aspects of the secondexample implementation further includes moving at least one of thenuclear waste canisters into a hazardous waste repository of thedirectional drillhole.

In another aspect combinable with any of the previous aspects of thesecond example implementation, each SNF canister comprises radiationshielding only on a top surface and a bottom surface of the SNFcanister.

In another aspect combinable with any of the previous aspects of thesecond example implementation, the radiation shielding comprisesconcrete.

In another aspect combinable with any of the previous aspects of thesecond example implementation, the lid is configured to move to exposethe top opening of the inner volume.

In another aspect combinable with any of the previous aspects of thesecond example implementation, the plurality of nuclear waste canisterscomprise at least fifteen nuclear waste canisters.

A third example implementation includes a method for forming adirectional drillhole for hazardous waste storage includes identifying asubterranean formation suitable to store hazardous waste; determiningone or more faults that extend through the subterranean formation;forming a vertical drillhole from a terranean surface toward thesubterranean formation; and forming a directional drillhole from thevertical drillhole that extends in or under the subterranean formationand parallel to at least one of the one or more faults. The directionaldrillhole includes a hazardous waste repository configured to store thehazardous waste.

An aspect combinable with the third example implementation furtherincludes forming the directional drillhole perpendicular to a directionof maximum horizontal stress of the subterranean formation.

In another aspect combinable with any of the previous aspects of thethird example implementation, the subterranean formation is located inan area of high risk of seismic activity.

In another aspect combinable with any of the previous aspects of thethird example implementation, the seismic activity includes earthquakes.

In another aspect combinable with any of the previous aspects of thethird example implementation, the hazardous waste includes radioactiveor nuclear waste.

In another aspect combinable with any of the previous aspects of thethird example implementation, the nuclear waste includes spent nuclearfuel or high level waste.

In another aspect combinable with any of the previous aspects of thethird example implementation, the subterranean formation includes atleast one of shale, clay, or salt.

In another aspect combinable with any of the previous aspects of thethird example implementation, the one or more faults includes at leasttwo parallel faults.

In another aspect combinable with any of the previous aspects of thethird example implementation, forming the directional drillhole includesforming the directional drillhole between and parallel to the at leasttwo parallel faults.

In another aspect combinable with any of the previous aspects of thethird example implementation, determining the one or more faultsincludes determining the one or more faults based on at least one of oneor more seismic records; one or more seismic reflection surveys; ageologic mapping; or one or more electromagnetic (EM) surveys.

In another aspect combinable with any of the previous aspects of thethird example implementation, the directional drillhole does not crossthe one or more faults.

In a fourth example implementation, a hazardous waste repository systemincludes a directional drillhole that extends from an entry proximate aterranean surface to a subterranean formation that includes one or morefaults that extend through the subterranean formation. The directionaldrillhole includes a substantially vertical portion coupled to theentry, a transition portion coupled to the substantially verticalportion, and a substantially horizontal portion that is coupled to thetransition portion and is formed in or under the subterranean formationand parallel to at least one of the one or more faults. The systemfurther includes a hazardous waste repository formed in thesubstantially horizontal portion of the directional drillhole; a storagecontainer positioned in the hazardous waste repository, the storagecontainer including an inner volume sized to enclose hazardous wastematerial; and a seal positioned in the directional drillhole thatisolates the hazardous waste repository from the entry.

In an aspect combinable with the fourth implementation, the directionaldrillhole extends perpendicular to a direction of maximum horizontalstress of the subterranean formation.

In another aspect combinable with any of the previous aspects of thefourth example implementation, the subterranean formation is located inan area of high risk of seismic activity.

In another aspect combinable with any of the previous aspects of thefourth example implementation, the seismic activity includesearthquakes.

In another aspect combinable with any of the previous aspects of thefourth example implementation, the hazardous waste material includesradioactive or nuclear waste.

In another aspect combinable with any of the previous aspects of thefourth example implementation, the nuclear waste includes spent nuclearfuel or high level waste.

In another aspect combinable with any of the previous aspects of thefourth example implementation, the subterranean formation includes atleast one of shale, clay, or salt.

In another aspect combinable with any of the previous aspects of thefourth example implementation, the one or more faults includes at leasttwo parallel faults.

In another aspect combinable with any of the previous aspects of thefourth example implementation, the directional drillhole extends betweenand parallel to the at least two parallel faults.

In another aspect combinable with any of the previous aspects of thefourth example implementation, the one or more faults are determinedbased on at least one of one or more seismic records; one or moreseismic reflection surveys; a geologic mapping; or one or moreelectromagnetic (EM) surveys.

In another aspect combinable with any of the previous aspects of thefourth example implementation, the directional drillhole does not crossthe one or more faults.

A fifth example implementation according to the present disclosureincludes a drillhole plug that includes a frame or housing comprising acorrosion-resistant material and sized to fit within a milled portion ofa directional drillhole that comprises a hazardous waste repository; anda material that fills at least a portion of the frame or housing. Thematerial exhibits creep such that the material fills one or more voidsbetween the frame or housing and a subterranean formation adjacent themilled portion of the directional drillhole.

In an aspect combinable with the fifth example implementation, themilled portion is located at a vertical portion of the directionaldrillhole.

In another aspect combinable with any of the previous aspects of thefifth example implementation, the milled portion does not include casingand other portions of the directional drillhole comprise casing.

In another aspect combinable with any of the previous aspects of thefifth example implementation, the material comprises a natural material.

In another aspect combinable with any of the previous aspects of thefifth example implementation, the natural material comprises a rockmaterial.

In another aspect combinable with any of the previous aspects of thefifth example implementation, the rock material comprises at least oneof shale, clay, or salt.

In another aspect combinable with any of the previous aspects of thefifth example implementation, the rock material is the same orsubstantially the same as the subterranean formation.

In another aspect combinable with any of the previous aspects of thefifth example implementation, the rock material is different than thesubterranean formation.

In another aspect combinable with any of the previous aspects of thefifth example implementation, an outer diameter of the plug is greaterthan an outer diameter of the directional drillhole.

In another aspect combinable with any of the previous aspects of thefifth example implementation, the outer diameter of the plug is lessthan a diameter of the milled portion.

A sixth example implementation according to the present disclosureincludes a method for sealing a drillhole that includes milling aportion of a directional drillhole that comprises a hazardous wasterepository; inserting a drillhole plug into the milled portion; andsealing the directional drillhole with the material of the drillholeplug that fills one or more voids between the frame or housing and asubterranean formation adjacent the milled portion of the directionaldrillhole. The drillhole plug includes a frame or housing that comprisesa corrosion-resistant material and a material that fills at least aportion of the frame or housing. The material exhibits creep.

In an aspect combinable with the sixth example implementation, themilled portion is located at a vertical portion of the directionaldrillhole.

In another aspect combinable with any of the previous aspects of thesixth example implementation, the milled portion does not include casingand other portions of the directional drillhole comprise casing.

In another aspect combinable with any of the previous aspects of thesixth example implementation, the material comprises a natural material.

In another aspect combinable with any of the previous aspects of thesixth example implementation, the natural material comprises a rockmaterial.

In another aspect combinable with any of the previous aspects of thesixth example implementation, the rock material comprises at least oneof shale, clay, or salt.

In another aspect combinable with any of the previous aspects of thesixth example implementation, the rock material is the same orsubstantially the same as the subterranean formation.

In another aspect combinable with any of the previous aspects of thesixth example implementation, the rock material is different than thesubterranean formation.

In another aspect combinable with any of the previous aspects of thesixth example implementation, an outer diameter of the plug is greaterthan an outer diameter of the directional drillhole.

In another aspect combinable with any of the previous aspects of thesixth example implementation, the outer diameter of the plug is lessthan a diameter of the milled portion.

A fifth example implementation according to the present disclosureincludes a power generator system that includes one or more heattransfer members configured to contact a heat source in a hazardouswaste repository of a directional drillhole that stores nuclear waste inone or more nuclear waste canisters and a heat sink in the hazardouswaste repository; and one or more thermoelectric generators thermallycoupled to the one or more heat transfer members and configured togenerate electric power based on a temperature difference between theheat source and the heat sink.

In an aspect combinable with the fifth example implementation, thenuclear waste comprises spent nuclear fuel.

In another aspect combinable with any of the previous aspects of thefifth example implementation, the heat source comprises at least one ofthe nuclear waste canisters or a casing disposed in the drillhole.

In another aspect combinable with any of the previous aspects of thefifth example implementation, the heat sink comprises at least one ofthe casing disposed in the drillhole or a material that at leastpartially fills the drillhole.

In another aspect combinable with any of the previous aspects of thefifth example implementation, the material comprises a liquid.

Another aspect combinable with any of the previous aspects of the fifthexample implementation further includes one or more biasing membersconfigured to urge the one or more heat transfer members into thermalcontact with the heat source and the heat sink.

Another aspect combinable with any of the previous aspects of the fifthexample implementation further includes at least one radiation shield.

In another aspect combinable with any of the previous aspects of thefifth example implementation, the radiation shield comprises tungsten.

In another aspect combinable with any of the previous aspects of thefifth example implementation, the one or more heat transfer memberscomprise a radiation resistant material.

In another aspect combinable with any of the previous aspects of thefifth example implementation, the one or more thermoelectric generatorscomprise a radiation resistant material.

A sixth example implementation according to the present disclosureincludes a method for generating power in a hazardous waste repositoryof a directional drillhole that stores nuclear waste that includescontacting, with one or more heat transfer members of a power generatorsystem, a heat source in the hazardous waste repository of thedirectional drillhole that stores nuclear waste in one or more nuclearwaste canisters; contacting, with the one or more heat transfer membersof the power generator system, a heat sink in the hazardous wasterepository; and generating electric power with one or morethermoelectric generators thermally coupled to the one or more heattransfer members based on a temperature difference between the heatsource and the heat sink.

In an aspect combinable with the sixth example implementation, thenuclear waste comprises spent nuclear fuel.

In another aspect combinable with any of the previous aspects of thesixth example implementation, the heat source comprises at least one ofthe nuclear waste canisters or a casing disposed in the drillhole.

In another aspect combinable with any of the previous aspects of thesixth example implementation, the heat sink comprises at least one ofthe casing disposed in the drillhole or a material that at leastpartially fills the drillhole.

In another aspect combinable with any of the previous aspects of thesixth example implementation, the material comprises a liquid.

Another aspect combinable with any of the previous aspects of the sixthexample implementation further includes urging, with one or more biasingmembers of the power generator system, the one or more heat transfermembers into thermal contact with the heat source and the heat sink.

Another aspect combinable with any of the previous aspects of the sixthexample implementation further includes shielding gamma rays generatedby the nuclear waste from the power generator system with at least oneradiation shield.

In another aspect combinable with any of the previous aspects of thesixth example implementation, the radiation shield comprises tungsten.

In another aspect combinable with any of the previous aspects of thesixth example implementation, the one or more heat transfer memberscomprise a radiation resistant material.

In another aspect combinable with any of the previous aspects of thesixth example implementation, the one or more thermoelectric generatorscomprise a radiation resistant material.

A seventh example implementation according to the present disclosureincludes a drillhole sealing system that includes a plurality ofdrillhole seals. Each drillhole seal includes a frame or housingcomprising a corrosion-resistant material and sized to fit within aparticular milled portion of a directional drillhole that comprises ahazardous waste repository; and a particular rock material that fills atleast a portion of the frame or housing. The rock material is selectedto match a geologic formation at a depth at which the particulardrillhole seal is set and exhibits creep such that the material fillsone or more voids between the frame or housing and the geologicformation at the depth and adjacent the particular milled portion of thedirectional drillhole.

In an aspect combinable with the seventh example implementation, eachparticular milled portion is located at a vertical portion of thedirectional drillhole.

In another aspect combinable with any of the previous aspects of theseventh example implementation, each particular milled portion does notinclude casing and other portions of the directional drillhole comprisecasing.

In another aspect combinable with any of the previous aspects of theseventh example implementation, the particular rock material comprisesat least one of shale, clay, or salt.

In another aspect combinable with any of the previous aspects of theseventh example implementation, an outer diameter of each drillhole sealis greater than an outer diameter of the directional drillhole.

In another aspect combinable with any of the previous aspects of theseventh example implementation, the outer diameter of each drillholeseal is less than a diameter of the particular milled portion.

In another aspect combinable with any of the previous aspects of theseventh example implementation, the particular rock material of one ofthe plurality of drillhole seals is different than the particular rockmaterial of another of the plurality of drillhole seals.

In another aspect combinable with any of the previous aspects of theseventh example implementation, a number of the plurality of drillholeseals matches a number of geologic formations between a terraneansurface and subterranean formation in which the hazardous wasterepository is formed.

In another aspect combinable with any of the previous aspects of theseventh example implementation, the particular rock material of each ofthe plurality of drillhole seals matches a respective geologic formationadjacent the drillhole seal.

In another aspect combinable with any of the previous aspects of theseventh example implementation, each frame or housing loosely containsthe particular rock material.

In another aspect combinable with any of the previous aspects of theseventh example implementation, the frame or housing comprises a wire ormesh enclosure.

An eighth example implementation according to the present disclosureincludes a method for sealing a drillhole that includes milling a firstportion of a directional drillhole that comprises a hazardous wasterepository and inserting a first drillhole plug into the first milledportion. The first drillhole plug includes a frame or housing thatcomprises a corrosion-resistant material and a first rock material thatfills at least a portion of the frame or housing. The first rockmaterial is selected to match a geologic formation at a depth at whichthe first drillhole plug is set and exhibits creep such that the firstrock material fills one or more voids between the frame or housing andthe geologic formation at the depth and adjacent the first milledportion of the directional drillhole. The method further includesmilling a second portion of the directional drillhole and inserting asecond drillhole plug into the second milled portion. The seconddrillhole plug includes a frame or housing that comprises thecorrosion-resistant material and a second rock material that fills atleast a portion of the frame or housing. The second rock material isselected to match a geologic formation at a depth at which the seconddrillhole plug is set and exhibits creep such that the second rockmaterial fills one or more voids between the frame or housing and thegeologic formation at the depth and adjacent the second milled portionof the directional drillhole. The method further includes sealing thedirectional drillhole with the first and second rock materials of therespective first and second drillhole plugs that fill one or more voidsbetween the frames or housings and a subterranean formation adjacent thefirst and second milled portions of the directional drillhole.

In an aspect combinable with the eighth example implementation, at leastone of the first or second milled portions is located at a verticalportion of the directional drillhole.

In another aspect combinable with any of the previous aspects of theeighth example implementation, each of the first and second milledportions does not include casing and other portions of the directionaldrillhole comprise casing.

In another aspect combinable with any of the previous aspects of theeighth example implementation, at least one of the first or second rockmaterials comprises at least one of shale, clay, or salt.

In another aspect combinable with any of the previous aspects of theeighth example implementation, an outer diameter of each of the firstand second drillhole plugs is greater than an outer diameter of thedirectional drillhole.

In another aspect combinable with any of the previous aspects of theeighth example implementation, the outer diameter of each of the firstand second drillhole plugs is less than a diameter of the respectivefirst and second milled portions.

In another aspect combinable with any of the previous aspects of theeighth example implementation, the first rock material is different thanthe second rock material.

In another aspect combinable with any of the previous aspects of theeighth example implementation, each frame or housing loosely containsthe respective first or second rock material.

In another aspect combinable with any of the previous aspects of theeighth example implementation, the frame or housing comprises a wire ormesh enclosure.

In another aspect combinable with any of the previous aspects of theeighth example implementation, the second drillhole plug is uphole ofand is in contact with the first drillhole plug.

Another aspect combinable with any of the previous aspects of the eighthexample implementation further includes milling a third portion of thedirectional drillhole; and inserting a third drillhole plug into thethird milled portion.

In another aspect combinable with any of the previous aspects of theeighth example implementation, the third drillhole plug includes a frameor housing that comprises a corrosion-resistant material and a thirdrock material that fills at least a portion of the frame or housing.

In another aspect combinable with any of the previous aspects of theeighth example implementation, the third rock material selected to matcha geologic formation at a depth at which the third drillhole plug isset.

In another aspect combinable with any of the previous aspects of theeighth example implementation, the third rock material exhibits creepsuch that the third rock material fills one or more voids between theframe or housing and the geologic formation at the depth and adjacentthe third milled portion of the directional drillhole.

Another aspect combinable with any of the previous aspects of the eighthexample implementation further includes further sealing the directionaldrillhole with the third rock material of the third drillhole plug thatfill one or more voids between the frame or housing and the subterraneanformation adjacent the third milled portion of the directionaldrillhole.

A number of implementations have been described. Nevertheless, it willbe understood that various modifications may be made without departingfrom the spirit and scope of the disclosure. For example, exampleoperations, methods, or processes described herein may include moresteps or fewer steps than those described. Further, the steps in suchexample operations, methods, or processes may be performed in differentsuccessions than that described or illustrated in the figures.Accordingly, other implementations are within the scope of the followingclaims.

What is claimed is:
 1. A power generator, comprising: a drillhole formedfrom a terranean surface into a subterranean formation; a hazardouswaste repository formed in the drillhole; one or more nuclear wastecanisters positioned in the hazardous waste repository, each of the oneor more nuclear waste canisters storing nuclear waste; one or more heatsource heat transfer members in at least thermal contact with a heatsource in the hazardous waste repository; one or more heat sink heattransfer members in at least thermal contact with a heat sink in thehazardous waste repository; one or more thermoelectric generatorsthermally coupled to the one or more heat source heat transfer membersand the one or more heat sink heat transfer members, the one or morethermoelectric generators configured to generate electric power based ona temperature difference between the heat source and the heat sink; andtwo or more springs configured to urge the one or more heat source heattransfer members and the one or more heat sink heat transfer membersinto contact with a casing positioned in the drillhole.
 2. The powergenerator of claim 1, wherein the nuclear waste comprises spent nuclearfuel.
 3. The power generator of claim 1, wherein the heat sourcecomprises at least one of the one or more nuclear waste canisters or thecasing positioned in the drillhole.
 4. The power generator of claim 1,wherein the heat sink comprises at least one of the casing positioned inthe drillhole or a material that at least partially fills the drillhole.5. The power generator of claim 4, wherein the material comprises aliquid.
 6. The power generator of claim 1, wherein the one or morenuclear waste canisters comprises at least two nuclear waste canistersseparated by a gap in the hazardous waste repository of four feet, andthe one or more thermoelectric generators is positioned in the gap, witha temperature at a midpoint of the gap being about 20° C. cooler than atemperature at either of the at least two nuclear waste canisters. 7.The power generator of claim 1, further comprising at least oneradiation shield.
 8. The power generator of claim 7, wherein the atleast one radiation shield comprises tungsten.
 9. The power generator ofclaim 7, wherein the at least one radiation shield comprises a firstradiation shield positioned at an end of at least one of the one or morenuclear waste canisters, and a second radiation shield positioned nearor in contact with the one or more thermoelectric generators.
 10. Thepower generator of claim 7, wherein the at least one radiation shieldcomprises a gamma ray shield.
 11. A method for generating power in ahazardous waste repository of a drillhole that stores nuclear waste,comprising: thermally contacting, with one or more heat source heattransfer members of a power generator, a heat source in the hazardouswaste repository of the drillhole that stores nuclear waste in one ormore nuclear waste canisters; contacting, with one or more heat sinkheat transfer members of the power generator, a heat sink in thehazardous waste repository; generating electric power with one or morethermoelectric generators thermally coupled to the one or more heatsource heat transfer members and the one or more heat sink heat transfermembers based on a temperature difference between the heat source andthe heat sink; and urging, with two or more springs, the one or moreheat source heat transfer members and the one or more heat sink heattransfer members into contact with a casing positioned in the drillhole.12. The method of claim 11, wherein the nuclear waste comprises spentnuclear fuel.
 13. The method of claim 11, wherein the heat sourcecomprises at least one of the one or more nuclear waste canisters or thecasing positioned in the drillhole.
 14. The method of claim 11, whereinthe heat sink comprises at least one of the casing positioned in thedrillhole or a material that at least partially fills the drillhole. 15.The method of claim 14, wherein the material comprises a liquid.
 16. Themethod of claim 11, wherein the one or more nuclear waste canisterscomprises at least two nuclear waste canisters separated by a gap in thehazardous waste repository of four feet, and the one or morethermoelectric generators is positioned in the gap, with a temperatureat a midpoint of the gap being about 20° C. cooler than a temperature ateither of the at least two nuclear waste canisters.
 17. The method ofclaim 11, further comprising shielding gamma rays generated by thenuclear waste from the power generator with at least one radiationshield.
 18. The method of claim 17, wherein the at least one radiationshield comprises tungsten.
 19. The method of claim 17, wherein the atleast one radiation shield comprises a first radiation shield positionedat an end of at least one of the one or more nuclear waste canisters,and a second radiation shield positioned near or in contact with the oneor more thermoelectric generators.
 20. The method of claim 17, whereinthe at least one radiation shield comprises a gamma ray shield.